Boiling vapor exhausting from prior art cryostats is usually discharged to the atmosphere at relatively low temperature in comparison with ambient conditions. That is, the cold escaping vapor is cool enough to transfer heat from the vent tube at such a rate as to lower the temperature of this structure to well below the freezing point of atmospheric moisture. This moisture condenses thereon, freezes and continues to accumulate. In proximity to the accumulating ice, there is necessarily a region cooled to an intermediate temperature, upon which moisture condenses and flows without freezing. Thus, even in this steady state, liquid is evolving. As a result, corrosion can appear on a cryostat and this is particularly dangerous because the neck tube is ordinarily welded to the body of the cryostat. Corrorion appearing at this point weakens the welded joint and may destroy the vacuum integrity of the cryostat internal structure. The neck tube(s) support much of the inner structure: severe corrosion may result in mechanical weakening of the same interior structure.
To reduce this accumulation, the prior art has resorted, in one example, to electrical means to supply heat to the escaping gas. Another approach has utilized wicking material to absorb moisture and transport same by capillary attraction to a reservoir. Still another approach can utilize very long venting tubes to afford additional surface for warming the vapor, but this can be unwieldy for filling the cryostat through inordinately long neck tubes.
It is an object of the invention to reduce the accumulation of ice and moisture on the exterior of the fill and vent tubes of a cryogenic container.
In the present invention a finned heat exchanger is disposed to supply heat derived from the atmosphere at ambient temperature to the boiling vapor traversing the interior of the fill and vent tube(s).